The Daily Life of the Inventor in Another World

The group of girls truly wanted to stay and observe Marcus at work. However… they had school! And with Serafall present, they couldn't afford to appear incompetent or neglect their responsibilities.

Especially since it was their own request to come to the human world in the first place! They needed to fulfill their duties, which meant leaving the genius inventor alone with the Satan herself.

It was the first day of human school for Rias and Sona, yet their minds were far from their classes.

All they could think about was the enigmatic figure downstairs, setting up a surveillance system for their town. Serafall's relentless praise of Marcus only fueled their curiosity. They had to see him in action.

The moment the final bell rang, the two rushed downstairs, eager to uncover what had been going on.

"What is that!?" Rias exclaimed, her wide eyes fixed on the object before her.

"It looks like a diamond," Sona replied, adjusting her glasses with a mix of curiosity and skepticism.

"I know it looks like a diamond, but diamonds don't glow or emit… particles like that," the redhead retorted, rolling her eyes.

"True. I'm just stating what I see," Sona said dryly with a shrug.

Their initial shock gave way to fascination as they ventured further into the room. It was unlike anything they had seen before—a strange fusion of a ritual chamber and a futuristic reactor.

Glass panels spun in perfect synchronization around the glowing diamond, seemingly absorbing the magical energy it emitted. The room itself was covered with intricate runes, carved into every surface—walls, ceiling, and floor.

The pair couldn't help but marvel at the sheer intricacy of the setup. Even devils, known for their mastery of rituals, would require far more time to create something of this scale. Yet, Marcus had achieved it all during the few hours they were in class.

"Umm… I thought barriers like this were magical? Why is there a… computer terminal?" Rias asked, her expression puzzled as she turned to Sona.

"Why are you asking me? You think I understand any of this?" Sona replied, her tone tinged with annoyance. She felt utterly out of her depth.

The younger Sitri struggled to process the sight before her. Among the runes were symbols from at least three distinct magical traditions, and she suspected there were others she couldn't identify. This level of knowledge far surpassed her own—and she'd spent most of her life in libraries!

"I thought you knew more than I did," Rias teased with a casual shrug.

In response, Sona hissed, a sound so cat-like it made Rias chuckle.

"No, I don't. This… this is beyond me," Sona admitted, shaking her head before scanning the room for Marcus.

She finally spotted him seated at a sleek desk, typing on what appeared to be a futuristic laptop. Several cables snaked out from the device, connecting it to the diamond-shaped barrier reactor.

Her attention then shifted to her older sister, who was perched casually on the desk, a book in hand. Serafall had been there the entire time.

Sona suppressed a groan. Of course, her sister was here. The Leviathan's apparent infatuation with Marcus was becoming increasingly obvious. The thought of the chaos this would cause in the Underworld sent a shiver down her spine.

What would the devils think if they learned one of their most powerful leaders was enamored with a human inventor?

"You two are here. Good. Come over—I'll explain some things," Marcus called, his calm voice cutting through their thoughts.

The pair froze momentarily before moving toward him.

Serafall glanced up briefly, giving them a smile before returning to her book. Sona couldn't tell if it was a reassuring smile or a mischievous one. Only time would tell.

"First, the barrier core," Marcus began, gesturing toward the glowing diamond. His tone was casual, as if he were describing something as mundane as a cup of coffee.

"It's officially called the Diamond Core Heart. I designed and created it myself. The core is reinforced with magical inscriptions for durability and has three protection layers. Since this one serves a slightly different purpose than my personal projects, the first two layers extend across the entire city."

Rias and Sona listened intently as Marcus continued.

"The first layer is surveillance. Magic spreads across the city, monitoring its population. If a certain threshold of magic is detected, the system reacts and notifies you. That's where the second layer—observation—comes into play. It identifies the source of the magic and its location. Finally, the third layer is protection, which can be manually activated from here. When engaged, it transforms the school into a fortress, capable of withstanding attacks even from Ultimate-Class Devils."

The room fell silent as the gravity of his explanation sank in.

"Where does it draw its power from? Surely, the Shinto Pantheon won't appreciate us draining Japan of magic," Sona asked, her voice laced with concern.

"It's powered by my M.T.M. technique," Serafall chimed in from her seat, twirling a strand of her hair absentmindedly.

"What does that stand for?" Sona inquired, her curiosity piqued.

"Motion to Magic," Serafall explained. "The core starts by converting motion into magic. Once initiated, the system enters a self-sustaining loop, generating magic indefinitely. It's essentially a perpetual magic reactor."

Sona's jaw dropped slightly.

"So you did pay attention," Marcus said, smirking at Serafall.

"Of course I did!" she shot back indignantly.

"It was hard to tell, considering you seemed to be spacing out," Marcus teased.

"Well, forgive me for being gobsmacked! You told me it would last as long as the planet!" Serafall exclaimed in a single breath.

"Fair point," Marcus said with a shrug.

Turning back to the pair, Marcus continued, "Now, for the part where I need your help."

He stood and approached the computer terminal.

"I'll add you two into the system. Right now, the core is operational only in this room. To extend its reach citywide, your peerage members will need to mark the boundaries of the barrier."

"Understood."

"I'll handle it."

Their synchronized responses pleased Marcus. Having watched the anime, he was well aware of their tendencies and how much trouble they could cause. Now, with constant surveillance, he could ensure they stayed on task.

He smirked inwardly at the thought. The moment they slacked off, he could report them directly to Venelana or Serafall.

"Good," he said simply, turning back to his terminal.

Rias whispered excitedly to Sona, "It's a touch screen!"

Sona nodded, her gaze fixed on the display, though she couldn't decipher the letters and numbers flashing across it. To her, it might as well have been another language.

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